Hydraulic systems are nowadays used for a plethora of different purposes.
One prominent example is the use of hydraulics for generating large forces. For this purpose, usually cylinders and pistons are used. Such devices are used, for example, in locks, steering systems, crawlers, forklift trucks, wheel loaders, and so on. Hydraulic systems for these types of machines are usually referred to as open-circuit hydraulics. This notation is used, because within the hydraulic actuator, for example in the hydraulic cylinder, a variable volume of hydraulic fluid is present. To compensate for these volume changes, a hydraulic fluid reservoir is provided. The hydraulic fluid reservoir is under atmospheric pressure and is usually built as a standard tank. To perform its function as a buffer for the hydraulic fluid, the tank usually has to be of considerable size. Since the hydraulic fluid in the reservoir is under atmospheric pressure, the hydraulic pump takes in hydraulic fluid directly from an atmospheric fluid reservoir. This is a main difference between open-circuit hydraulic systems and closed-circuit hydraulic systems, which are described in the following.
Another application where hydraulic components became very popular are transmissions for vehicles which benefit from continuous variable ratio and wheelspeed combined with high tractive effort over the whole speed range and especially at low speeds. Such transmissions very often use closed-circuit hydraulic pumps and closed-circuit hydraulic motors. The hydraulic motor converts the high-pressure energy of the hydraulic fluid into mechanical energy and sends the hydraulic fluid, now at a lower pressure level, back to the hydraulic pump. Such a system is generally referred to as closed-circuit hydraulics, because the hydraulic pump is sending and receiving almost the same flow rate of hydraulic fluid under all working conditions of the hydraulic circuit. Therefore, no buffer is needed. The low pressure side of such systems normally operates between 10 and 30 bars. Because of this closed-circuit systems normally have fewer problems with filling of the hydraulic pump than open-circuit hydraulic systems.
In real applications, however, even a closed-circuit hydraulic system still has some hydraulic fluid reservoir under atmospheric conditions. First of all, leakage of hydraulic fluid has to be considered. Especially in devices with mechanically moving parts, such as in hydraulic pumps and hydraulic motors, fluid leaks can never be totally avoided. The leakage fluid is therefore collected and transferred to the fluid reservoir via collecting lines. The collected hydraulic fluid is pumped back into the closed-circuit hydraulic system (normally to the low-pressure side of the circuit) by means of a charge pump. Sometimes, a small fraction of hydraulic fluid is taken out of the closed hydraulic circuit for cooling and filtration purposes. This is commonly referred to as “loop flushing”. A pressure relief valve and/or an orifice take out a certain percentage of the total fluid flow rate on the low pressure side of the closed-circuit hydraulic system. This flush part of the fluid flows through a heat exchanger and heat can be transferred from the hydraulic fluid to the ambient air. Having passed the heat exchanger and optionally a fluid filter, the fluid is ejected to the hydraulic fluid reservoir. From there, it is pumped back to the main fluid circuit by means of a charge pump, together with the leakage hydraulic fluid. The fraction of hydraulic fluid, used for cooling and filtration purposes, is relatively small and is lower than about 20 percent of the fluid flow rate in the main hydraulic circuit.
While hydraulic systems perform well in practice, they are still undesirably large and expensive for certain applications.
Especially in open-circuit hydraulic systems, problems arise in high performance conditions. Under such high performance conditions the hydraulic pump has to deliver a large flow rate of hydraulic fluid. This, of course, requires the hydraulic pump to receive an appropriate amount of hydraulic fluid from the fluid reservoir. To be able to do this, the suction line of the hydraulic fluid pump has to have a huge cross section, so that a sufficient fluid supply rate to the hydraulic fluid pump can be provided and the pressure drop can be kept low. However, not only the suction line has to have a large cross section, but also the fluid inlet port (e.g. the valve plate of an axial piston machine) of the hydraulic pump needs to be designed with a sufficiently large cross-section. These requirements for large supply cross sections result in relatively large sizes of pump and motor parts, fittings, flanges, hoses and pipes and hence of the overall size of the resulting hydraulic system. This leads to increased costs for the manufacture and use of such hydraulic systems, especially when considering the increased volume requirements in the machine or vehicle, where the hydraulic system is used.
In check ball pump designs the inlet check valve always means an additional flow restriction and the aforementioned problem increases. Normally this results in limited fill speed of such pumps. Very often the inlet valve is actually held close by a spring and the fluid has to work against the spring. The pump has to suck the inlet valve open. Synthetically commutated hydraulic pumps are very similar to check ball pumps when considering the aforementioned problem. In such synthetically commutated hydraulic pumps, also known as digital displacement pumps (which are a unique subset of variable displacement pumps), the fluid valves do not open passively under the influence of pressure differences. Instead, the fluid valves are actively controllable by appropriate valve actuating units which are controlled by an electronic control unit. Even when the inlet valve in a synthetically commutated hydraulic pump is of the normally open type, it provides additional inlet flow restriction which limits fill speed when the pump takes in hydraulic fluid from an atmospheric hydraulic fluid reservoir.
These synthetically commutated hydraulic pumps fall into two groups. In the first group, only the inlet valve is actively controlled, whereas the fluid outlet valve remains passive. With this type, a full stroke pumping mode, a partial stroke pumping mode and a no-pumping mode can be obtained. With the second type, where both inlet and outlet valves are of the actively controllable type, a full or partial stroke back pumping mode/motoring mode can be realised as well. This is known in the state of the art.
The requirement of a large supply cross-section is a major drawback for synthetically commutated hydraulic pumps. Not only valve cross-sections, and therefore the valve head in the valve channel, have to be of large size, but also the valve actuating unit has to be able to deliver a sufficiently large force as well as a sufficiently large travel. This, in turn, increases the costs for such a hydraulic pump. Moreover, the driving unit of the valve has high power consumption. This increases the costs for the manufacture and the actual use of such a hydraulic system even further. On off-highway mobile equipment for instance this would require the installation of large and expensive alternators to generate sufficient electrical power for inlet valve actuation.